Get the Audio Commentary patch, last updated for FM 2016 here:
Audio Commentary Patch for FM16
1) Copy "match_events.xml" and "match_sounds.cfg" from the commentary patch to your installation folder: "Football Manager 2018\data\simatch\events"
2) a) Use the "resource archiver" and unpack the file "simatchviewer.fmf" which can be found in "Football Manager 2018\data".
b) Save the extracted folders in a separate directory.
c) All folders in the directory you leave "as is" only the "sounds" folder you empty and replace the files with the ones in the commentary audio downloaded file. make sure there are no subfolders in it (I remember there is a "default" subfolder for sure), so all audio files should be directly put into the (now empty) "sounds" folder.
Once done you use Resource Archiver again but now you pack all the files in that folder (not only the sounds folder, but all the files you extracted earlier) into a new "simatchviewer.fmf" and put the files in the folders I indicated.
d) Use the "resource archiver" to create a new "simatchviewer.fmf" file from the changed folder with "sounds" folder replaced with the commentary contents in it.
e) Move the "simatchviewer.fmf" file to the installation folder: "Football Manager 2018\data" and replace the existing file.
Make sure your game isn't running while you do all this and restart the game, empty cache and reload it, don't forget to allow sounds in matches to be played in "preferences".
Reload the cache and play your Football Manager 2018 game with live commentary again!
I tested this and works already for weeks without any issues.
ALWAYS make a backup of you original files in case something goes wrong or you want to revert back to the original sounds